1. Field of the Invention
The present invention relates to an exhaust gas purification device for purifying harmful components in an exhaust gas discharged from an internal combustion engine, a combustion device, or the like.
2. Description of the Related Art
An exhaust gas discharged from an internal combustion engine such as an engine or a combustion device generally contains therein nitrogen oxide (NOx), carbon monoxide (CO), hydrocarbon (HC), and the like as harmful components. As a related-art exhaust gas purification device for purifying an exhaust gas generated by stoichiometric combustion in which the amount of the residual oxygen (O2) in the exhaust gas is very small, a device using a three-way catalyst has gone into practical use. Contact of the exhaust gas discharged from an engine with the three-way catalyst converts the harmful components contained in the exhaust gas into harmless components such as water (H2O), carbon dioxide (CO2), and nitrogen (N2).
In recent years, in engines for cars, attention has focused on a lean-burn engine having a system for combustion with a gasoline in a smaller amount than usual in order to improve the fuel efficiency. The lean-burn means lean combustion. The exhaust gases discharged from an internal combustion engine such as the lean burn engine or a diesel engine, or the exhaust gases from industrial combustion devices, household fan heaters, or the like contain therein a large amount of oxygen. Thus, with the foregoing method using a three-way catalyst, it is difficult to purify nitrogen oxide (NOx) which is one of harmful components.
As the method for purifying such an exhaust gas containing a large amount of oxygen, for example, there is disclosed an exhaust gas purification device including a first three-way catalyst part, a plasma discharge part, and a second three-way catalyst part from upstream of a discharge gas passage (e.g., see JP-A-7-317535 (pages 3 and 4, FIG. 1)). Whereas, as another method, there is disclosed a discharge gas purification device including an oxidizing catalyst part, a plasma discharge part, and a NOx occlusion reduction catalyst part from upstream of a discharge gas passage (e.g., see JP-A-2002-256853 (page 3, FIG. 1)). Further, there is disclosed a discharge gas purification device including a plasma discharge part, a first reduction purification catalyst part, and a second reduction purification catalyst part from upstream of a discharge gas passage (e.g., see JP-A-2004-305841 (page 3, FIG. 1)).
However, with the related-art exhaust gas purification device including a first three-way catalyst part, a plasma discharge part, and a second three-way catalyst part from upstream of a discharge gas passage, when oxygen is contained in a large amount in the discharge gas, the oxidation reaction with O2 of a reducing agent proceeds more preferentially than the reduction reaction of NOx at the first three-way catalyst part using platinum or rhodium as a catalyst. Accordingly, most of the hydrocarbons serving as a reducing agent required for purification of NOx at the plasma discharge part are consumed. As a result, to the plasma discharge part present at downstream of the catalyst parts, hydrocarbons to be a reducing agent are not supplied. Therefore, at the plasma discharge part, OH* (OH radical) and O* (O radical) having a high oxidizing power are generated by the plasma reaction of oxygen or water vapor in the exhaust gas. Thus, unfavorably, it becomes difficult to reduce and purify NOx only by the plasma reaction (in the formula, * means that atoms or molecules are in an excited state).
Whereas, with the related-art discharge gas purification device including an oxidizing catalyst part, a plasma discharge part, and a NOx occlusion reduction catalyst part from upstream of a discharge gas passage, hydrocarbons are oxidized and consumed at the oxidation catalyst part at the uppermost stream part. Thus, hydrocarbons to be a reducing agent are not supplied to the plasma processing part situated at downstream thereof. Thus, at the plasma discharge part, OH* (OH radical) and O* (O radical) having a high oxidizing power are generated by the plasma reaction of oxygen or water vapor in the exhaust gas. Thus, unfavorably, it becomes difficult to reduce and purify NOx only by the plasma reaction.